Out of various types of solar cells, compound solar cells including a light absorbing layer formed of a CIS or CIGS (hereinafter referred to as “CIGS type”) compound semiconductor are known to be advantageous in that they can be produced in the form of a thin film as having a higher conversion efficiency and are less liable to suffer from reduction in conversion efficiency due to light irradiation or the like.
Conventionally, such a solar cell has a buffer layer formed by chemically depositing Zn(O,S) or the like (see PLT1). Where the buffer layer is formed by the chemical deposition method, however, the solar cell is produced by performing a light absorbing layer forming step in a vacuum, once taking out the resulting product to an atmospheric environment to perform a buffer layer forming step, and performing a front side electrode layer forming step again in a vacuum. Therefore, these steps cannot be sequentially performed, resulting in reduction in productivity. To cope with this, it has been proposed to form the buffer layer by a sputtering method rather than by the chemical deposition method so that these steps can be sequentially performed in a vacuum to increase the productivity (see PLT2). On the other hand, it is known that, where a difference ΔEc in conduction band between the light absorption layer and the buffer layer is set to 0≤ΔEc≤0.4, the recombination of carriers around the interface between the light absorbing layer and the buffer layer can be efficiently suppressed to provide a higher conversion efficiency (see NPL1). Further, where the CIGS type compound semiconductor is used for the light absorbing layer, exemplary materials for the buffer layer having a conduction band satisfying the aforementioned condition include mixed crystal compounds such as Zn(O,S,OH), In(S,OH) and (Zn,Mg)O.